It's a jungle Not to mention in how many ways it has been interpreted outside of the Far East traditions.

I have studied the concept some, also comparing it with similar concepts in other cultures. Such a term, used for some or other kind of life force, also related to air, breathing, wind, you find in many traditions in the world. Prana in India, pneuma i Greece, ruah in Semitic traditions, and so forth.

Yes, and some of these are directly related. The problem is that all these terms reflect ancient attempts to explain how things work.... and yet none of these explanations were sustainable truths as man's knowledge progressed, regardless of whether such terms were used in olden times.

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I guess that at the bottom of it, they deal with the old mysteries of what is the difference between being alive and being dead (and what happens to that which made someone alive, when he has died), what makes man and animals able to move, and what makes them will to move, et cetera.

A mountain can have ki. A place can have ki. It is not always about life and death.

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has an interesting etymology in its kanji, consisting of the sign for vapor/fume/air and that for rice. Boiling rice. To anyone in China and Japan it's a very relevant symbol of life energy. You gotta eat.
You could also call it a symbol for that in the air, which you need to stay alive.

Or it could simply be the symbol for pressure. Like the pressure from breathing. How pressure and strength work together in the universe. Not surprisingly, it is common to refer to ki as "air pressure" in a number of instances. Without air pressure you can die, of course. If your pressure is adequate, you don't get diseases very easily. There are a number of approaches, I think.

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Anyway, is in the core of aikido, and maybe one's understanding of ki develops just like aikido as a whole does: by training.

I agree with that, more or less. However, as is evident from the huge number of people currently doing very external and incomplete arts (of the ones that use ki), years of practice doesn't necessarily give one complete understanding. I.e., just training isn't everything. It's an interesting discussion and it relates back to a previous discussion of why so much information is not shown, IMO.